Random Number Generator
There are many modern computerized slot machines. So the odds are whatever they are programmed to be. In modern slot machines, the reels and lever are present for historical and entertainment reasons only. The positions of the reels depend on a Random Number Generator. There is a Random Number Generator in the machine's software.
The RNG generates random numbers at an extremely high rate. The most recent random number is used to determine the result when the Play button is pressed. This means that the result varies depending on exactly when the game is played. The result changes every fraction of a second.
The numbers generated by the RNG seem to be not exactly random. It turned out that The number sequence is repeated time after time. This behavior is due to poor programming. It is relatively easy to build pseudo RNGs with very long periods. A single period can't be completed by any computer in the expected lifetime of the universe. The access to the pseudo RNG code and seed values was known to Ronald Dale Harris, a former slot machine programmer. He discovered equations for specific gambling games like Keno. They allowed to predict that the next set of selected numbers would be based on the previous games played. However, this is impossible for most machines, because the RNG picks numbers even when the machine is not being played. So the player cannot see the secquence.

Payout Percentage
Winnings 82–98 per cent of the money that is wagered by players are typically paid out by slot machines. This is known as the theoretical payout percentage. Jurisdictions serve to control the minimum theoretical payout percentage. Law or regulation typically establish it. A certain winning pattern is determined by every casino individually. It contains the information about the amounts they pay and the frequencies of these pay-outs. The main criterion for selecting the winning patterns on slot machines is that they should yield a certain fraction of the money played to the house. The players get the rest of the money.
A slot machine's theoretical payout percentage is set at the factory when the software is written. Changing the payout percentage requires a physical swap of the software. Based on current technology, this is a time-consuming process and as such is done infrequently. Certain jurisdictions presuppose the EPROM to have a tamper-evident seal. It can only be changed in the presence of Gaming Control Board officials. Other jurisdictions randomly audit slot machines to ensure that they contain only approved software.
The Nevada Gaming Commission is working now with Las Vegas casinos on technology that would allow the casino's slot manager to change the game, the odds, and the payouts remotely. The change can be done only after the selected machine has been idle for at least four minutes. The machine must be locked to new players for four minutes and display an on-screen message informing potential players that a change is being made after the change is made.

Machines That Are Linked
Often machines are linked together in a way that allows a group of machines to offer a particularly large prize, or jackpot. Each slot machine in the group contributes a small amount to this progressive jackpot, awarded to a player who gets a specific combination of symbols. The amount paid for the progressive jackpot is usually far higher than any single slot machine could pay on its own.
Sometimes multiple machines form multiple casinos. In these cases the manufacturer owning these machines is responsible for paying the jackpot.

Near-miss
The reel display of modern slot machines is under computer software's control. That is why it is possible to make the slot machine frequently display combinations that are close to winning combinations.
The term near-miss programming is used to denote this practice.
There is a related phenomenon that is also sometimes called near-miss. The chance of a winning combination appearing on a pay line is controlled by the winning percentages programmed into the slot machine. However, the combinations appearing above and below the pay line are all roughly equally randomly distributed. This means it is much more likely that a winning combination will appear above or below a pay line than on the pay line. This occurs if abbreviated physical reels are used to display a win pattern based upon the RNG. In video slot machines the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
There was an investigation held by the Nevada Gaming Commission concerning the issue of a near-miss above or below the pay line. If the near-miss above or below the pay line was not specially programmed it was stated to be legal. That means that the possibility of near-miss occurrence should be equal to the possibility of any other combination. It is impossible to program the machine to show winning combinations more frequently than other combinations above or below the pay line.
In Australia the usage of near-miss programming, where a near miss is inaccurately displayed is also illegal. Manufacturer's practices are audited by regulators with the help of stop motion cameras.

Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to cheating devices and other scams.
Modern slot machines are controlled by EPROM computer chips and coin acceptors were changed for bill acceptors. These machines and their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. Nowadays microwaves are used to defraud slot machines.